Tester for collision-detect circuitry

ABSTRACT

A tester for the collision detector of a transceiver for a multiple access data communications network using carrier-sense collision detection for controlling access to the network. A squelch circuit is employed in the transceiver&#39;s transmitter for enabling and disabling the transmitter output. An end-of-transmission detector monitors the squelch circuit to detect the termination of a transmission. Upon termination of a transmission, a collision simulator circuit supplies to the transceiver&#39;s receiver a signal of predetermined amplitude and duration, to simulate the input the receiver gets when a collision actually occurs. If the receiver is working properly, it signals a collision to the host (i.e., computer, terminal, etc). If it does not receive the collision signal at the appointed time, the host knows that either the transceiver&#39;s collision detection circuit or the tester is not working.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of my co-pending applicationSer. No. 267,394, filed May 26, 1981, titled Transceiver for LocalNetwork Using Carrier Sense Multiple Access/Collision Detection.

This application also relates to the following commonly assignedco-pending applications:

Ser. No. 292,004, filed herewith, titled Reliability Enhancements ForTransceivers For Local Data Networks Using Carrier Sense MultipleAccess/Collision Detection, Ser. No. 292,005, filed herewith, titledCurrent Source Transmitter Output Stage For Local Data Networks and Ser.No. 292,006, filed herewith, entitled Precision Setting of Currents andReference Voltages.

FIELD OF THE INVENTION

This invention relates to the field of data communications, and, moreparticularly, to a transceiver for a multi-access communications systemwhich employs carrier sense multiple access/collision detectiontechniques for distributed control.

BACKGROUND OF THE INVENTION

In multi-access communication systems, or networks, for interconnectingmultiple devices or stations in a communications network, means must beprovided for controlling access of the devices to the communicationschannel. It has previously been proposed to provide a network using asingle coaxial cable to interconnect devices for communication with eachother. Of course, only one device can transmit at any given time oversuch a cable, if all devices operate at the same frequency. One form ofcontrol which has evolved for such networks is referred to ascarrier-sense multiple access with collision detection ("CSMA/CD"). Withthis technique, each device controls its own access to the coaxial cablechannel. Each device which uses the channel interfaces to the cablethrough a transceiver which includes apparatus for transmitting a signalonto the channel as well as apparatus for receiving a signal from thechannel placed thereon by another device's transceiver. The transceiverseach include a collision detector for generating a collision signalwhenever a signal transmitted on the cable by another transceiver isdetected at the same time the transceiver itself is transmitting ontothe channel. Each transceiver sends the collision signal back to itshost device and in response both hosts stop transmitting. Both thenretry transmission after the channel is clear.

Each device that wants to use (i.e., transmit on) the channel, first"listens" to the receiver in its transceiver to hear if any otherstation is transmitting. If it detects no other host transmitting, thestation starts its transmission, while receiving at the same time.Another station also might start transmitting, though. If that happens,both detect the collision and stop, as noted above. To avoid repeatedcollisions, each then waits a pseudorandom interval and tries again. Oneach retry, the pseudorandom delay is increased.

Such a system is illustrated, for example, in U.S. Pat. No. 4,063,220,issued Dec. 13, 1977 to Robert M. Metcalfe et al.

In such CSMA/CD networks, care must be taken to ensure that the failureof one transceiver or one host device does not impair the usability ofthe network by other stations. Several transceiver failure modes can beforeseen as "bringing down" the network. Among these is a collisiondetector failure. If a transceiver cannot properly detect collisions, itmust not be used to transmit.

Accordingly, it is an object of the present invention to provide animproved transceiver for use in such networks, with greatly enhancedreliability and protection against recognizable failure modes whichwould adversely affect the network.

It is a further object of the invention to provide a transceiver havingintegral means for simulating collisions to test the collision mechanismin its receiver, to insure its proper operation.

Yet another object of the invention is to provide a collision-detecttester which is invisible to other stations in the network.

These and other, further objects, features and advantages of the presentinvention will be understood from the following description.

SUMMARY OF THE INVENTION

In accordance with this invention, a transceiver for use with a CSMA/CDsystem utilizing a coaxial cable channel incorporates circuitry forguarding against the possibility of transceiver failure interfering withnetwork operation. The protection circuitry includes means for testingthe collision detection circuits in the transceiver by simulating acollision at the end of every transmission. It further includes meansfor isolating the transceiver from the coaxial cable so that shortcircuits in the transceiver do not short circuit the coaxial cable.Guard circuitry also is provided in the transceiver's transmitter, toprevent the transmitter from becoming stuck on and monopolizing thecoaxial cable channel. The guard circuitry includes a timer which turnson switches to short out the transmitter output a predetermined timeafter the start of each transmission.

The collison detector tester monitors the transmitter's squelch signal;at the end of a transmission, that squelch signal changes state. Thischange of state is detected and used to turn on a circuit whichgenerates a signal simulating a collision. In turn, the collisionsimulation signal is fed to the receiver. If the receiver is operatingproperly, it detects the "collision" and so signals the host. The host,however, knows that the collision detected right after the end of atransmission is merely the result of a test. Conversely, if no collisionis signalled, the host knows the test has been failed; it can then takeappropriate action to avoid and prevent use of the transmitter until thecollisiondetect circuit is repaired or the transceiver is replaced.

To facilitate assembly and reduce cost, the transceiver is implementedin emitter-coupled logic. This, however, creates compliance problems forthe current source which the transmitter uses as a cable driver.Emitter-coupled logic (ECL) generally requires a -5 volt power supply.When a current source is made from ECL, there is a limit to how low theoutput voltage can be pulled before the circuit becomes a voltage sourceinstead of a current source. This point is referred to as the compliancelimit of the source. Typically, an ECL current source has a two andone-half volt compliance limit relative to the supply voltage; that is,it cannot be pulled down more than two and a half volts before it willcease to behave as a current source. In a CSMA/CD system, that creates aproblem since the transmitter output must be able to swing about 6-7volts due to the presence of a collision and the voltage drop across thetransmitter output isolation circuit. Conventional circuit design wisdommost likely would suggest that emitter-coupled logic could not be used,in view of this problem. Operation of the ECL current source between-5.2 and -10.2 volts, rather than the conventional range of 0 to -5volts, however, solves this problem, when the receiver input circuitryand the collision detect circuitry are operated on the -5.2 volt supply.

Another way to state the problem is that this arrangement provides ameans to ensure that the output current source does not saturate whentransmitting into the cable at the same time as another transmitter'ssignal is present. If it did, the collision would not be detected, asthe cable voltage would not be lowered enough to trigger collisiondetection.

A further feature of the invention is that the current source generatesa staircase waveform instead of an abrupt transition when changinglogical states. This reduces the high frequency components in thetransmitter's output and therefore reduces the size of the filtercapacitors needed, thereby minimizing phase distortion of thezero-crossings.

This invention is pointed out with particularity in the appended claims.A more thorough understanding of the above and further objects andadvantages of this invention may be obtained by referring to thefollowing detailed description, which should be read in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the transmitter portion of the transceiverof the present invention;

FIG. 2 is a block diagram of the receiver portion of the transceiver ofthe present invention;

FIG. 3 is a block diagram of the power supply for the transceiver of thepresent invention;

FIG. 4 is a schematic circuit diagram of the signal-generating portionof the transmitter of FIG. 1;

FIG. 5 is a schematic circuit diagram of the guard circuitry 25 of FIG.1;

FIG. 6 is a schematic circuit diagram of the transmitter squelch,end-of-transmission detector and collision-detect simulator blocks ofFIG. 1;

FIG. 7 is a schematic circuit diagram of the collision detect, receivesquelch and threshold setting portions of the receiver of FIG. 2;

FIG. 8 is a schematic circuit diagram of the band-pass filter andamplifier portions of the receiver of FIG. 2;

FIG. 9 is a schematic circuit diagram of the d.c.-to-d.c. converter ofFIG. 3; and

FIG. 10 is a schematic circuit diagram of the power supply regulator ofFIG. 3.

DETAILED DESCRIPTION OF AN ILLUSTRATED EMBODIMENT

Referring now to FIG. 1, there is shown a block diagram of thetransmitter portion of the transceiver of the present invention. Thecomputer or other host device (not shown) which communicates via thetransceiver supplies information to be transmitted over lines 12A and12B to the primary winding of an input transformer 14. The secondarywinding of transformer 14 feeds a buffer amplifier 16 and a transmittersquelch circuit 18. Buffer 16, in turn, feeds a cable driver circuit 20.

The cable driver 20 is a current source which generates the transmitsignal applied to the coaxial cable 22 through an isolation circuit 24.

The transmitter squelch circuit 18 monitors the secondary winding oftransformer 14 to determine whether the host is sending any signal fortransmission. When no signal is being supplied, transmitter squelch 18disables cable driver 20 so that no spurious information will betransmitted.

Isolation circuit 24 prevents a short circuit in the transmitter fromshorting out the cable. As will be apparent from the discussion below,other circuitry in the transceiver intentionally may short out thetransmitter or a component failure may do so.

To protect against the possibility of either the cable driver 20 or thehost getting stuck on in an active, transmitting mode (which would, ofcourse, prevent other transceivers from gaining access to the cable 22),one or more guards circuits 25 are placed across the output of cabledriver 20. The purpose of guard circuits 25 is to short circuit theoutput of cable driver 20 after the time allotted for a transmissionpacket, so that the transmitter does not continue to transmit into thenetwork.

Although a single guard circuit should be adequate, multiple guardcircuits may be employed for redundancy and high reliability. If morethan one guard circuit is used, all are wired in parallel.

Each guard circuit includes a guard switch 26 and a guard switch timer28. Each guard switch is separately controlled by an individual guardswitch timer 28; all of the guard switch timers, however, are controlledby a single guard driver circuit 32.

The rules of such a network (i.e., its communications protocol) normallydefine a maximum transmission packet length, at least partially for thepurpose of insuring that one user does not monopolize the channel. Ifthe host device at a particular station violates this rule for somereason, or the cable driver sticks on and transmits noise or a d.c.level after the host has stopped supplying a message, guard switchtimers 28 and guard switches 26 provide a means for disconnecting theinvolved transceiver from the network so that other users are not undulyinconvenienced. This isolates the problem to the station which hascaused it. To this end, guard driver 32 uses the transmitter squelchsignal as an indicator of transmitter activity and starts the guardswitch timers running at the start of each transmission. After the timedefined by the guard switch timers 28, the guard switches 26 are turnedon, shorting the output current of cable driver 20 to ground and thuseffectively removing the transmitter from the network.

The transceiver is further provided with circuitry for testing itscollision detection circuitry. This is done by a collision detect ortest mechanism 34. The collision detector test mechanism functionallycomprises an end-of-transmission detector 36 and a collision simulator38. The end-of-transmission detector 36 monitors the transmitter squelchsignal for a change of state indicating the termination of atransmission, at which time it actuates the collision simulator 38. Thecollision simulator feeds a signal into the receiver section of thetransceiver (at point A, as indicated in FIG. 2). If the collisiondetection circuitry in the receiver is operating properly, it willsignal a collision to the host device. Due to the timing of thecollision signal relative to the end of the transmission, the hostcomputer knows to recognize it as a validation of the test. Conversely,it knows that if the collision signal does not appear, the test wasfailed and the transceiver should not be used for transmission until itis repaired.

The receiver section of the transceiver is illustrated in FIG. 2, inblock diagram form. As shown there, the receiver 40 interfaces with thenetwork cable 22 through an isolation circuit 42. There are two signalprocessing paths in the receiver, both fed from the output of theisolation circuit 42. The first path includes a band pass filter 44, anamplifier 46 and an output transformer 48. The second path includes alow-pass filter 52, a collision detector 54 and a 10 MHz oscillator 56which is controlled by the collision detector 54 to signal collisiondetection. The output of the oscillator 56 feeds a transformer 58through which it signals the host. (All signals to or from the host mustbe transformer-coupled due to the need for electrical isolation amonghosts.) A receiver squelch circuit 62 interconnects the two paths, andgates on the receiver only in the presence of a valid signal.

Low-pass filter (LPF) 52 time-averages the received signal, to provide ad.c. signal indicative of whether information is being transmitted onthe cable 22. The output of the collision simulator 38, since it mimicsthe presence of more than one transmitter signal on the line, issupplied as a second input to LPF 52. Collision detector 54 is athreshold detector which monitors the output of low pass filter 52. Ifthe LPF output exceeds a predetermined threshold, corresponding to thepresence of more than one active transmitter, a collision is indicatedand the 10 MHz oscillator 56 is then turned on to signal collisiondetection.

Receiver squelch circuit 62 also responds to the output of LPF 52 byproviding a squelch signal to amplifier 46. The squelch signal turns offamplifier 46 when the output of the LPF 52 is so low as to indicate thatno signal is being received.

It will be appreciated that the proper operation of collision detector54 and receiver squelch circuit 62 require the use of stable, accuratethresholds. They are provided by a threshold setting network 64. Inaddition to the threshold voltages supplied to the collision detectorand receiver squelch, the threshold setting network 64 also provides afeedback signal (at point B) whose value is proportional to thethreshold signal. This feedback signal is sensed by a comparisionamplifier 72 in the power supply 70 (shown in FIG. 3), to control one ofthe two power supply output voltages.

Power supply 70, in FIG. 3, employs a d.c.-to-d.c. converter 74 togenerate a -10.2 volt d.c. supply from a host power supply of +9.4 to+15.75 volts. In addition, a shunt regulator 76 provides a -5.2 voltsupply derived from the -10.2 volt source. The shunt regulator 76 iscontrolled by a comparison amplifier 72 which compares a voltageestablished by zener diode 78 with the feedback voltage generated atpoint B in the threshold setting network 64 of FIG. 2. Thus, the actualoutput level of the nominally -5.2 volt supply is adjusted and regulatedso as to maintain the feedback voltage at point B equal to the voltageestablished by zener diode 78. Consequently, the output of regulator 76may be greater or less than -5.2 volts, as required to provide precisionsetting of the threshold by the threshold setting network 64.

FIG. 4 shows the basic circuitry for the transmitter of the transceiver.The signal to be transmitted is applied to the primary winding (i.e.,leads 12A and 12B) of a transformer 14. The secondary winding oftransformer 14 provides the input of a buffer amplifier, or linereceiver 16. A pair of matched resistors 76A and 76B are also connectedin series across the secondary winding of transformer 14, for impedancematching. A center tap is provided at the junction of the two resistors76A and 76B, and is labelled point C. Point C is connected to asimilarly labelled point on FIG. 6, at the output of the squelchcircuit. The non-inverting output of buffer amplifier 16, on line 94,provides the signal input to the cable driver 20.

Cable driver 20 is a current source which may be turned on and off todrive the coax cable channel with a binary waveform. The output fromcable driver 20, however, is not simply a square wave type of output. Ithas been found desirable to provide a staircase ramp rather than anabrupt leading and falling edge transition. Thus, from a square waveinput, the cable driver 20 generates a well-controlled symmetrictrapezoid waveform, through a two-pole RC network which gives a 25 nsrise-time. This gets rid of much of the high frequency energy in thebinary waveform without phase-distorting the zero crossings. If a squarewave had been used, rather than a staircase, larger filter capacitorswould have been needed, thus causing unacceptable phase distortion.

In the exemplary embodiment in FIG. 4, cable driver 20 is implementedwith four stages, 78A-78D, providing four steps to the rising andfalling staircase. Of course, a different number of steps also can beemployed; the number of stages to be used is at the designer'sdiscretion, and four has simply been found convenient. Each of thestages is a differential current switch having a signal input,non-inverted and inverted outputs, and a control (i.e., enable/disable)input. Except for the signal inputs, the current switches 78A-78D areconnected in parallel, with like terminals connected to each other.Their non-inverted outputs are all connected to a resistors 82A-82C,which connect to ground, and to one lead of a first filter capacitor 84.The inverted outputs of current switches 78A-78D are connected to theother lead of capacitor 84, which connection also provides the cabledriver output, at point (or node) D. There are four other connections tonode D, as well: (1) a second filter capacitor 86, connected betweennode D and the transceiver ground; (2) a series string of three diodes92A-92C connected between node D and the coaxial cable, comprisingisolation circuit 24; (3) guard switches 26, connected from node D toground; and (4) a transmit bias current switch, shown in FIG. 6.

The enable/disable terminals of the current switches 78A-78D receive anenable/disable signal at point C, from the transmitter squelch circuitof FIG. 6.

In isolation circuit 24, three diodes are connected in series to protectagainst the possibility of one, or even two diodes short circuiting.Should point D be grounded, diodes 92A-92C become back-biased (opencircuits) during other hosts' transmissions, thereby isolating thetransmitter from the network. Conversely, the sole result of an opendiode is to cause single station failure, while leaving the networkintact. The isolation network provides important protection, because thetransceiver is direct coupled to the network cable.

The operation of cable driver 20, when enabled, is as follows: Atransition from zero to one on line 94, the asserted output of buffer16, turns on current switch 78A (the first stage of the ramp generator)and also is supplied to the input of a delay line 96 and begins topropagate therethrough. Delay line 96 is tapped three times at, forexample, 6 nanoseconds, 12 nanoseconds and 18 nanoseconds of delay. The6 nanosecond delay provides the input to the second stage current switch78B, to turn it on 6 nanoseconds after current switch 78A was turned on.Similarly, the 12 nanosecond tap is connected to the input of thirdstage 78C and the 18 nanosecond output drives the fourth stage 78D.Because the current switches 78A-78D are connected in parallel and drawcurrent through the same load, they provide additive outputs, with theamplitude of the output signal increasing each time another one of thoseswitches is turned on in their machine-gun firing sequence, therebyproviding the staircase signal. Resistors 82A-82C and capacitors 84 and86 provide a two-pole RC filter for smoothing the output signal.

Referring now to FIG. 5, guard circuits 25A-25C for shorting the outputof the cable driver 20 are shown. The guard circuits comprise aplurality of switches 26A-26C connected in parallel, each having oneterminal connecting to the transceiver ground and the other terminalconnected to node D. The switches are all controlled by a common driver32 (gate 108 and transistor 112); and the driver is operated by a signalsupplied at point E, which indicates the start of each transmission.

In the exemplary embodiment, the transceiver uses three guard circuits25A, 25B and 25C, connected in parallel. The guard circuits areidentical to one another. The number of guard circuits employed is atthe designer's discretion, since multiple guard circuits merely provideredundant protection and reduce the statistical probability offunctional failure in the transceiver which "brings the network down."When three guard circuits are employed, of course, the chances of themall being out of commission is much lower than the chance for one guardcircuit alone being out of commission.

Since all of the guard circuits are identical, only guard circuit 25Awill be explained in detail, it being understood that the sameexplanation applies with respect to the other guard circuits.

Guard circuit 25A comprises a guard switch 26A and a guard switch timer28A, which includes transistors 102A, 104A and 106A, plus theirassociated components. Guard switch transistor 26A is connected as asimple common emitter switch with its emitter attached to thetransceiver ground and its collector attached to the node D of FIG. 4.When actuated, the switch transistor 26A shorts out node D to ground. Itis open only when there is a valid signal to transmit. A guard circuitdriver comprising OR/NOR gate 108 and transistor 112 controls theoperation of guard switch timer 28A.

A "stuck on" current switch 78A-78D will draw current from the guardswitch(es) rather than the network.

When the signal at connection point E from the transmitter squelchcircuit of FIG. 6 changes state to indicate the beginning of atransmission (i.e., it goes high), transistor 112 emitter goes low anddiode 116 becomes back biased, allowing resistor 122 to dischargecapacitor 118. Resistor 122 controls the rate at which capacitor 118discharges. When capacitor 118 is discharged sufficiently, the voltageacross it turns off transistor 104A. Transistors 104A and 106A are anemitter-coupled pair, so when transistor 104A turns off, transistor 106Aturns on, activating switch 26A and driving transistor 26A intosaturation. Thus, a predetermined time after the beginning of a "stuck"transmission, all of the guard switches are turned on, shorting theoutput of cable driver 20 and effectively disconnecting it from thecable. At the end of an "unstuck" transmission, transistor 112's emittergoes high, turning switch 26A on, also recharging capacitor 118 viadiode 116 in preparation for a subsequent transmission.

Turning now to FIG. 6, there is shown the remainder of the transmittercircuitry, including squelch and other control circuits.

Amplifiers 152 and 154 plus the associated componentry form the transmitsquelch circuit. This is a conventional type of squelch circuit whoseoperation will be readily understood from the drawing by those skilledin the art. Accordingly, a detailed explanation is unnecessary to theunderstanding of this invention. The squelch circuit monitors thetransmit signal from the host (which appears across points 14-A and14-B, the secondary leads of transformer 14) and provides two outputs,the cable driver enable/disable signal which is provided on line 156(point C) and the complement to that signal which is provided on line158.

Transistors 162 and 164 comprise an emitter-coupled pair. The base oftransistor 162 is driven by the nonasserted output of amplifier 154, online 156. The collector of transistor 162 supplies an output signal atconnection point E, which connects to a similarly labelled point on FIG.4 to drive the guard circuit driver. The signal at point E isessentially an inverted and level-shifted representation of the signalon line 156.

Transistor 164 is driven at its base by the asserted output of amplifier154, on line 158. The collector of transistor 164 provides a transmitbias which is connected to point D on FIG. 4. During transmission,transistor 162 is turned off and transistor 164 is turned on.

The signal on line 158 from amplifier 154 also feeds transistor 172through capacitor 174 and resistors 176 and 178. At the end of atransmission, when the squelch changes state, a spike or pulseapproximately one microsecond wide is produced at the emitter oftransistor 172 due to the effect of differentiation produced bycapacitor 174. This turns on transistor 182 for a like duration andproduces a collision test signal at the collector of transistor 182,which is shown as a point F to be connected to a similarly marked pointon FIG. 7, in the low-pass filter of the receiver.

The collision test signal simulates the effect of a collision by drawingexcess current through the receiver input at the end of every packettransmitted. Transistor 204 isolates the collision test signal from thenetwork, so it is not seen by other stations; only the transmittingstation responds to it. Further, this collision test does not sufferfrom "single point of failure" problem. The host, of course, must checkat the end of each transmission for the presence of the "collisionsignal" generated by the collision test signal.

Attention is now directed to the aforesaid FIG. 7, which illustrates thelow-pass filter, receiver squelch, collision detector, collisionoscillator and threshold setting circuits. Connection point H is thereceiver input from the coaxial cable (and, of course, the transmitteroutput, as well). It is attached to a fusible resistor 202 whichprovides protection against a short circuit in transistor 204 having anyadverse impact on the network. Transistor 204 is connected as a unitygain emitter-follower. A current source comprised of transistor 206 andresistor 208 biases transistor 204 so that it is always conducting.

Transistor 214 and the associated components (capacitors 216, 218 and222 plus resistors 224, 226 and 228) provide an emitter-follower circuitconfigured as a three-pole low-pass filter. The output of the low passfilter, on line 232, represents the average value of the signal on thecable, offset by the base-emitter voltage drops of transistors 204 and214. The signal on line 232 is compared against two thresholds. Onethreshold is used to determine whether there is a single signal on thecable (in which event the receiver squelch should be turned off) andanother threshold is used to determine whether there are two or moresignals on the cable (in which event a collision is indicated). Thesethresholds are applied, respectively, to the inverting input 234 of adifferential line receiver which is used as a comparator 236 and to theinverting input 238 of a similar comparator 242. The signal on line 232is supplied to the non-inverting input 244 of comparator 236 and,through a resistor 246 to the non-inverting input 248 of comparator 242.

The two thresholds are set by a precision current source comprisingtransistor 252 plus a compensation circuit comprising transistors 258and 262. The base of transistor 252 is connected to the -5.2 volt supplyand its emitter is connected to the -10.2 volt supply through a resistor264. Resistor 264 is a high precision resistor; the voltage across itsets the emitter current of transistor 252. The power supply adjusts theprecise value of the regulted output (which is nominally -5.2 volts) toachieve the desired current through resistor 264. To this end, thevoltage at the emitter of transistor 252 is sensed by the power supplyand a connection to the power supply is shown at a point labelled B.Transistor 258 is connected as a similar current source with an emitterresistor 266 having the same value as resistor 264, to produce the sameemitter current.

Transistor 262 is the load for current source 258 and its sole purposeis to draw a base current into transistor 262 which will raise thecurrent drawn through resistors 268 and 272 by an amount which verynearly equals the difference between the collector current and emittercurrent of transistor 252. Thus, transistor 262 is intended tocompensate for the fact that transistor 252 has a finite current gain asa result of which its collector current does not exactly equal itsemitter current, by pulling a current through resistors 268 and 272which is very close to the emitter current through resistor 264.

Preferably, transistor 252 and 262 will be provided on the samesubstrate so as to provide matching current gain, and thereby set thecurrent in resistors 268 and 272 exactly equal to the emitter current oftransistor 252. The actual threshold voltages are developed acrossresistors 268 and 272 plus the sum of the base-emitter drops acrosstransistors 254 and 256. Transistors 254 and 256 are used to match thecorresponding base-emitter drops of transistors 214 and 204.

Preferably, transistors 256 and 204 will be provided on the samesubstrate so that their base-emitter voltage drops will match andtransistors 254 and 214 will also be provided on the same substrate, forthe same reason.

The signal provided on line 232, to be compared with the referencethresholds, is produced by filtering the sum of the signal received overthe coaxial cable and the collision simulation test signal supplied atpoint F.

The comparator 236 drives a buffer amplifier 274, the output of whichprovides the receiver squelch signal at point K, which is connected tothe similarly labelled point in FIG. 8.

The comparator 242 detects whether the collision threshold has beenexceeded. In turn, it drives a buffer amplifier 276 which turns on andoff a 10 MHz oscillator formed by OR/NOR gate 278, capacitors 282 and284, inductor 286 and resistor 288. The OR/NOR gate 278 provides abalanced output for driving a transformer 292 through which a collisionpresence signal is supplied to the host device.

Turning now to FIG. 8, the receiver circuitry will be explained indetail. The receiver gets its input at point L, the emitter oftransistor 204 in FIG. 7. Resistor 302 and capacitor 304 filter out thehigh frequency components in the received signal. Transistor 306 is anemitter follower which acts as a buffer and provides a low impedanceoutput to drive a high pass filter comprising capacitor 308 and resistor312, on the non-inverting input of a differential line receiveramplifier 314.

The output of amplifier 314 feeds another buffer amplifier stage 316.The receiver squelch signal from amplifier 274 (connection point K) isalso connected to the output of buffer amplifier 316. Since this isemitter-coupled logic, if the output of either amplifier 274 oramplifier 316 is high, then their common connection (point K) is high.Thus, the output of amplifier 316 is squelched, or turned off, bymaintaining the output of amplifier 274 high.

The output of amplifier 316 also feeds the non-inverting input ofanother amplifier stage 318, which then feeds an OR/NOR gate 320. Gate320 provides gain and also presents a balanced differential outputsignal to drive a transformer 322 through which the receive signal iscommunicated to the host device.

In FIG. 9, circuitry is provided for a d.c.-to-d.c. converter useful inthe power supply of the present invention for converting +9.4 to +15.75volt power from the host or other source to -10.2 volts. The details ofthe d.c.-to-d.c. converter are unimportant in terms of understanding thepresent invention; other converter circuits could be employed just aswell and the design of power supplies, including the design of suchconverters, is well-developed in the electrical arts. Thus, for purposesof this explanation, it is sufficient to note that the d.c.-to-d.c.converter 74 receives a positive voltage at terminal M relative to aninput power return, or ground, at terminal N and that it supplies anoutput voltage at a terminal P relative to an isolated transceiverground at terminal R. A co-operating voltage regulator shown in FIG. 10,is connected to terminals P and R. This voltage regulator also suppliesa feedback signal to a point S in the converter, responsive to which thed.c.-to-d.c. converter adjusts its output voltage.

Turning now to FIG. 10, the regulator circuitry is shown for taking theunregulated voltage supplied at terminal P, regulating it at -10.2 voltsand providing a further regulated output of -5.2 volts at point T.

A 4.3 volt zener diode 78 is connected across the 10.2 volt supplythrough a series 590 ohm resistor 358, to provide a precise referencepoint 4.3 volts above (i.e., more positive than) the nominally -10.2volt supply, at the non-inverting input 359 of operational amplifier360. A precision resistive divider network 362 and 364 is designed toprovide 4.3 volts at the inverting input 366 of amplifier 360 when thevoltage across terminal points R and P is precisely 10.2 volts. Theoutput of amplifier 360 provides a feedback signal to the d.c.-to-d.c.converter 74, at point S. The feedback signal supplied to point Srepresents the error between the 4.3 volt reference provided by zenerdiode 78 and the voltage measured at the inverting input of amplifier360, and, thus, the amount by which the voltage at terminals P, Rdiffers from 10.2 volts. The d.c.-to-d.c. converter 74 responds to thefeedback signal at point S by adjusting its output so as to eliminateany error signal.

The 4.3 volt signal across zener diode 78 is also applied to thenon-inverting input of comparision amplifier 72, as a reference for the-5.2 volt supply. Unlike the -10.2 volt supply, however, the output ofthe -5.2 volt supply is not forced to bear a definite relationship tothe zener diode voltage. Rather, a particular point in the thresholdcircuit (powered from the -5.2 volt supply) is forced to have a definiterelationship to that voltage. The inverting input of amplifier 72receives the signal generated at point B of FIG. 7, the current sourcefor the threshold-setting network. The output of amplifier 72 controls ashunt regulator comprised of transistors 372 and 374, as well ascapacitors 376, 378, 382 and 384, to provide a regulated -5.2 voltoutput at point T. The resulting precise regulation is at point B, whereit has direct effect on the precision of threshold-setting.

Amplifier 72 senses a point remote to the power supply itself, where aspecific voltage (equal to the zener diode voltage) should appear; itcompares the sensed voltage with the reference voltage established bythe zener diode and drives the shunt regulator output to force theremote, monitored voltage to equal the reference voltage. Any deviationbetween the two produces an error signal which causes either transistor372 or 374 to be driven harder, thus shifting the electrical position oftheir emitter connections relative to the 10.2 volt supply.

This circuit thus permits a single zener diode reference to be used toregulate two different supply voltages, one in the power supply and theother in one of the circuits powered by the supply.

Having thus described the invention, it will be apparent that variousmodifications, alterations, and improvements will readily occur to thoseskilled in the art. Accordingly, it is intended that such alterations,modifications and improvements as are obvious herefrom be includedwithin the scope of this invention. The invention is intended to belimited only as defined in the appended claims, the foregoingdescription being illustrative only, and not limiting.

Having illustrated and described my invention, I claim:
 1. In atransceiver for connecting a host device to a multiple access datacommunications network of the type employing carrier-sense collisiondetection control techniques, and wherein the transceiver includes atransmitter and a receiver, the transmitter including a squelch circuitfor providing a squelch signal to control the enabling and disabling ofthe transmitter output, and the receiver including means for detectingcollisions--i.e., multiple transmitters simultaneously transmitting, theimprovement comprising:(a) end-of-transmission detector means responsiveto the squelch signal, for providing an end-of-transmission signal atthe conclusion of a transmission; and (b) collision simulation means,responsive to the end-of-transmission signal, for providing to thereceiver a signal simulating a collision, whereby the means fordetecting collision is tested automatically at the end of everytransmission.
 2. The apparatus of claim 1 wherein the signal provided tothe receiver by the collision simulator means is a pulse ofpredetermined amplitude and duration.